The primate model of hemiparkinsonism produced by the intracarotid infusion of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), allows the study of neurochemical changes associated with lesions of the substantia nigra pars compacta. Quantitative autoradiography after in vitro radioligand binding with [3H]Sch23390 (D1 antagonist) and [3H) sulpiride (D2 antagonist) were used to determine the distribution of receptor sites in MPTP-induced hemiparkinsonian monkeys. Alterations in the distribution of both Dl and D2 receptor binding sites in the caudate and putamen on the side of the lesion were associated with significant increases In Bmax as compared to the untreated hemisphere, but not with changes in Kd. MPTP lesions then, not only cause alterations in D2 receptor population density, but in Dl receptors as well.